The present invention generally relates to interference cancellation, and more particularly relates to cancelling inter-symbol interference in OFDM signals.
Orthogonal Frequency Division Multiplexing (OFDM) is a digital multi-carrier modulation scheme utilizing multiple closely-spaced, orthogonal sub-carrier frequencies. Each sub-carrier is modulated with a conventional modulation scheme (e.g., quadrature amplitude modulation) at a low symbol rate, maintaining data rates similar to conventional single-carrier modulation schemes in the same bandwidth. OFDMA allows several users to share the available bandwidth by allocating different sub-carriers to the different users, making the users orthogonal to one another. The allocation of sub-carriers may be dynamic, such as allocating a larger number of sub-carriers to users that have a larger amount of data to transmit. OFDM is deployed or planned for a variety of wireless systems, including IEEE 802.16 (WiMAX), some IEEE 802.11a/g wireless LANs (Wi-Fi), IEEE 802.20 Mobile Broadband Wireless Access (MBWA), and the like.
One challenge for OFDM involves inter-symbol interference (ISI). The presence of ISI in a signal received at a receiver interferes with the receiver's ability to recover the transmitted symbols from the received signal. One form of ISI comprises ISI between symbols transmitted on different sub-carriers at the same time (within the same OFDM block), referred to herein as inter-channel ISI. Inter-channel ISI occurs, e.g., when orthogonality between the sub-carriers is lost during transmission. Another form of ISI comprises ISI between symbols transmitted on the same sub-carrier at different times (between different OFDM blocks), referred to herein as inter-block ISI. Inter-block ISI occurs, e.g., when the transmitted symbols of one OFDM block spread in time and overlap with the transmitted symbols of another OFDM block.
Still another form of ISI, referred to herein as diagonal ISI, comprises a combination of both inter-channel and inter-block ISI, where ISI occurs between symbols transmitted on different sub-carriers at different times. Diagonal ISI has two primary components: forward diagonal ISI and reverse diagonal ISI. Forward diagonal ISI represents the effect of the combined inter-channel and inter-block ISI caused by past symbols on current symbols. Reverse diagonal ISI represents the effect of the combined inter-channel and inter-block ISI caused by future symbols on current symbols.
Wireless providers have expended significant effort in compensating for ISI in wireless OFDM systems. Some conventional systems use receiver processing techniques that collectively reduce inter-block, inter-channel, and diagonal ISI. However, such processing techniques are undesirably complex and typically require a significant amount of processing power.
In other systems, pre-filtering transmission symbols using a carefully selected pulse-shaping filter may reduce and possibly remove inter-channel and/or inter-block ISI. Such pre-filtering techniques are significantly simpler and require less power than previous ISI compensation techniques. For example, a pulse-shaping filter having Nyquist properties in the time or frequency domain may be used to reduce inter-block or inter-channel ISI, respectively. Filters with frequency-domain Nyquist properties reduce inter-channel ISI by controlling the spectral spread of the different sub-carrier symbols in the same OFDM block. An exemplary frequency domain Nyquist filter is defined by a function having a Fourier Transform with a root-raised cosine shape and a square with a raised cosine shape. Filters with time-domain Nyquist properties reduce inter-block ISI by controlling the time spread of the OFDM blocks. An exemplary time domain Nyquist filter comprises an isotropic orthogonal transform algorithm (IOTA) pulse, such as disclosed in U.S. Pat. No. 7,103,106, which is incorporated herein by reference.
Further, a pulse-shaping filter defined by a function with Nyquist properties in both the time and frequency domains may be used to simultaneously reduce inter-channel and inter-block ISI. Such filters simultaneously control the spectral and time spread. Exemplary functions that generate such pulse-shaping filters having Nyquist properties in both the time and frequency domains are described in U.S. Provisional Patent Application Ser. No. 60/924,673 to applicant filed 25 May 2007, which is incorporated in its entirety herein by reference. An exemplary function with Nyquist properties in both the time and frequency domains is a function having the Nyquist property in one domain, where the function is also its own Fourier Transform.
Even though Nyquist pulse-shaping filters reduce and/or remove inter-channel and/or inter-block ISI, such filters do not address diagonal ISI. As such, there remains a need for reducing diagonal ISI.